"A C preprocessor macro can be used to detect these two cases. Usually, when libintl.h was found and not explicitly disabled, the ENABLE_NLS macro will be defined to 1 in the autoconf generated configuration file (usually called config.h). In the two negative situations, however, this macro will not be defined, thus it will evaluate to 0 in C preprocessor expressions. "
http://www.gnu.org/software/gettext/manual/gettext.html